Does black coffee break a fast?

No. Black coffee, plain tea and water contain essentially no calories and do not meaningfully interrupt a fast. Coffee with milk, cream, sugar, butter or MCT oil does, because all of those carry calories. If your aim is simply eating less over the day, a splash of milk matters far less than the total.

Which protocol should I start with?

16:8, almost always. Sixteen fasted hours sounds severe until you realise that seven or eight of them are spent asleep, so in practice it means skipping breakfast or finishing dinner earlier. Longer fasts add difficulty faster than they add results, and the schedule you keep doing beats the aggressive one you drop after a fortnight.

Will I lose muscle while fasting?

Not if you keep protein and resistance training up. The risk comes from squeezing the window so tight that total protein falls short, which is most common on 20:4 and OMAD. Aim to spread protein across the meals you do eat, and train close to the window rather than at the far end of a long fast.

Can I exercise while fasted?

Many people train fasted without any problem, particularly for easy or moderate sessions. High intensity or long sessions often feel noticeably harder late in a fast, and performance can drop. If a session matters, put it near the start of your eating window or shortly after your first meal rather than at hour fifteen.

Who should not try intermittent fasting?

Anyone pregnant or breastfeeding, anyone under 18, anyone with a history of disordered eating, and anyone who is underweight. People with type 1 diabetes or taking insulin or sulfonylureas need medical supervision because skipping meals can cause hypoglycaemia. If you take regular medication with food, check with your doctor or pharmacist first.

Is intermittent fasting better than counting calories?

Not inherently. Trials generally find similar weight loss when the two produce the same energy deficit. The real difference is adherence: some people find a single timing rule far easier to follow than tracking everything, and others find a fixed window socially impossible. The better method is the one you will still be using in six months.

Intermittent Fasting Calculator

Why use this calculator

Most people who try intermittent fasting give up because the schedule collides with their life rather than because the approach failed. Written down as clock times against your own day, the decision becomes obvious: a 16:8 window starting at midday means no breakfast and a normal family dinner, while a window starting at 8 AM means breakfast and an early last meal.

This calculator turns the protocol name into those exact times, shows how many hours a week you would actually be fasted, and rates the difficulty honestly so you can pick something you will still be doing in three months. Fasting is a timing tool, not a magic one.

It works mainly by making it harder to overeat, and the protocol you can sustain beats the aggressive one you abandon.

How to use it

Choose a protocol first.
The number before the colon is the fasted hours and the number after it is the eating window, so 16:8 means sixteen hours fasted and an eight hour window, while OMAD compresses everything into roughly one hour.
Then enter the hour you want to start eating on a 24 hour clock, so 12 for midday or 8 for 8 AM, and the calculator fills in the rest of your day.
Finally select sex, which is only used to set the calorie target on 5:2 restriction days, where the commonly used figures are 500 for women and 600 for men.
Try the start hour at a couple of settings before committing, because the difference between eating at 10 AM and at 1 PM decides whether the schedule survives contact with work and family meals.

What each result means

Protocol: The fasting pattern the rest of the results are built from, either a daily window or the weekly 5:2 pattern.
Eating Window: The clock hours in which all food is eaten, which is the only part of the schedule you have to remember.
Fasting Window: The hours with no calories at all, running from your last meal to the next day's first one.
Break Your Fast At: The time your first meal is due. Starting with protein rather than sugar keeps the rest of the day steadier.
Stop Eating At: The time the window closes. Everything after this, including snacks and milky drinks, counts as breaking the fast.
Hours Fasted Each Week: Total fasted hours across seven days, which is a fairer comparison between protocols than the daily figure.
Fasting Difficulty: How demanding this schedule tends to be in practice, based on how much of the fast falls outside sleep.
Normal Eating Days: On 5:2, the five days a week you eat normally at maintenance rather than restricting anything.
Restriction Days: On 5:2, the two non consecutive days a week when intake is capped at the target shown below.
Restriction Day Calorie Target: The commonly used 5:2 cap for restriction days, 500 calories for women and 600 for men.
Suggested Restriction Days: A workable pairing that keeps restriction days apart and leaves weekends and social meals alone.
Estimated Weekly Calorie Deficit: Roughly how far below maintenance the two restriction days put you across a week.

The formula

Eating window hours = 24 minus the fasted hours in the protocol, so 16:8 gives 8 hours and 20:4 gives 4

Stop eating time = start eating time plus the eating window, wrapped around the 24 hour clock

The fasting window runs from the stop time to the next day's start time.

Weekly fasted hours = fasted hours multiplied by 7

For the 5:2 protocol there is no daily window: you eat normally on five days and cap intake at about 500 calories for women or 600 for men on two non consecutive days, which produces a weekly deficit of roughly 3,000 to 4,000 calories against typical maintenance intakes.

How this number is used in practice

In everyday use, intermittent fasting is a portion control strategy wearing a clock. Compressing eating into eight hours removes late night snacking and, for most people, one meal's worth of calories, which is where the results come from. The research picture is consistent with that: trials generally show time restricted eating produces weight loss similar to conventional calorie restriction, with the advantage that some people find one simple rule easier to follow than counting everything.

If you eat the same amount inside the window as you did across the whole day, the scale will not move.

The practical failure points are predictable. Windows that exclude a family dinner rarely survive. Very short windows make it genuinely hard to reach a sensible protein intake, which matters if you are trying to keep muscle while losing fat. Training hard at the far end of a long fast leaves many people flat.

And fasting is not for everyone: it is not appropriate during pregnancy or breastfeeding, for anyone with a history of disordered eating, for people who are underweight, or for people with type 1 diabetes or on medication such as insulin or sulfonylureas that can cause hypoglycaemia without food. Talk to your doctor before starting if any of that applies to you.

Worked examples

16:8 starting at midday, the standard beginner setup
Eat 12:00 PM to 8:00 PM, fast the other 16 hours, which is 112 fasted hours a week.
18:6 starting at 1 PM for someone who skips breakfast anyway
Eat 1:00 PM to 7:00 PM, fast 18 hours a day and 126 hours a week.
OMAD with the single meal in the evening
Eat 6:00 PM to 7:00 PM only, fasting 23 hours a day and 161 hours a week.
5:2 for a woman who prefers two hard days to a daily window
Five normal days plus two days capped at 500 calories, roughly 3,000 to 4,000 calories below maintenance for the week.

When people use it

Setting an eating window that still allows a normal family dinner
Comparing 16:8 and 18:6 as clock times before committing to either
Planning meal prep around a fixed window rather than around the whole day
Working out where training sessions sit relative to the fast
Deciding whether a daily window or the weekly 5:2 pattern suits your schedule

Tips

Water, black coffee and plain tea do not break a fast. Milk, sugar and juice do.
Start at 16:8 and widen the fast later only if the schedule is comfortable, not because a longer one sounds better.
Break the fast with protein and fibre rather than something sweet, which steadies appetite for the rest of the window.
Check you can still reach your protein target inside the window, especially on 20:4 and OMAD.
Put the window where your social meals already are, since a schedule that cancels dinner will not last.
Expect one to two weeks of hunger and headaches while you adapt, and keep fluids and salt up during it.
Fasting does not override total intake. If weight is not moving, the window is not the problem.

What it assumes, and where it stops being accurate

Calorie intake inside the window is assumed to be sensible. Fasting alone does not create a deficit.
The 5:2 deficit estimate assumes maintenance intake of roughly 2,000 to 2,500 calories, which varies widely by body size and activity.
Times are shown on a plain 24 hour clock with no adjustment for shift work, travel or daylight saving changes.
It is not appropriate during pregnancy or breastfeeding, for under 18s, or for anyone with a history of disordered eating.
It is not safe without medical supervision for people with type 1 diabetes or on insulin or sulfonylureas.
Individual response varies. Sleep, stress and training load all change how a given fasting window feels.

Terms used on this page

Eating window: The block of hours in which all food is eaten on a time restricted eating protocol.
Time restricted eating: Limiting food to a set number of hours each day without necessarily changing what is eaten.
OMAD: One meal a day, effectively a 23 hour fast with a single eating hour.
5:2: Eating normally on five days and capping intake near 500 or 600 calories on two non consecutive days.
Maintenance intake: The daily calorie level at which body weight stays stable, the baseline any deficit is measured against.

Questions people ask

Does black coffee break a fast?: No. Black coffee, plain tea and water contain essentially no calories and do not meaningfully interrupt a fast. Coffee with milk, cream, sugar, butter or MCT oil does, because all of those carry calories. If your aim is simply eating less over the day, a splash of milk matters far less than the total.
Which protocol should I start with?: 16:8, almost always. Sixteen fasted hours sounds severe until you realise that seven or eight of them are spent asleep, so in practice it means skipping breakfast or finishing dinner earlier. Longer fasts add difficulty faster than they add results, and the schedule you keep doing beats the aggressive one you drop after a fortnight.
Will I lose muscle while fasting?: Not if you keep protein and resistance training up. The risk comes from squeezing the window so tight that total protein falls short, which is most common on 20:4 and OMAD. Aim to spread protein across the meals you do eat, and train close to the window rather than at the far end of a long fast.
Can I exercise while fasted?: Many people train fasted without any problem, particularly for easy or moderate sessions. High intensity or long sessions often feel noticeably harder late in a fast, and performance can drop. If a session matters, put it near the start of your eating window or shortly after your first meal rather than at hour fifteen.
Who should not try intermittent fasting?: Anyone pregnant or breastfeeding, anyone under 18, anyone with a history of disordered eating, and anyone who is underweight. People with type 1 diabetes or taking insulin or sulfonylureas need medical supervision because skipping meals can cause hypoglycaemia. If you take regular medication with food, check with your doctor or pharmacist first.
Is intermittent fasting better than counting calories?: Not inherently. Trials generally find similar weight loss when the two produce the same energy deficit. The real difference is adherence: some people find a single timing rule far easier to follow than tracking everything, and others find a fixed window socially impossible. The better method is the one you will still be using in six months.

Use this with

Intermittent Fasting Calculator answers one part of the question. These pick up where it stops: TDEE Calculator, Calorie Calculator, Macronutrient Calculator and BMR Calculator.

Where these figures come from

Works anywhereLast reviewed 2026-09·How we build and check calculators

This calculation is jurisdiction-neutral arithmetic. No country-specific rules are applied.

NHS, Healthy weight. Weight management and safe calorie deficits
Harvard T.H. Chan School of Public Health, The Nutrition Source on intermittent fasting. Evidence review of fasting protocols
National Institute on Aging, Calorie restriction and fasting diets. What the research does and does not show

Intermittent Fasting Calculator produces an estimate for general information. It is not financial, tax, medical, legal or engineering advice. read the full disclaimer. Spotted a wrong number? Report it and it gets fixed. The form picks up this calculator and the inputs you used.